Electrolyte for electrochemical energy storage devices

ABSTRACT

An electrolyte for an electrochemical storage device is disclosed. In one embodiment, the electrolyte includes a lithium salt from about 3% to about 20% by weight, a primary solvent from about 15% to about 25% by weight, wide-temperature co-solvents from about 14% to about 55% by weight, interface forming compounds from about 0.5% to about 2.0% by weight, and a flame retardant compound from about 6% to about 60% by weight. The electrolyte interacts with the positive and negative electrodes of the electrochemical storage device to provide both high performance and improved safety such that the electrolyte offers adequate ionic conductivity over the desired operating temperature range, a wide electrochemical stability window, high capacities for both the cathode and anode, low electrode-electrolyte interfacial resistance, and reduced flammability.

PRIORITY STATEMENT & CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Patent Application No.61/990,308, entitled “Electrolytes with Reduced Flammability and WideOperating Temperature Ranges” and filed on May 8, 2014, in the names ofChristopher P. Rhodes and Matthew E. Mullings; which is herebyincorporated by reference for all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with government support under contractsN68936-09-C-0059, N68335-10-C-0347, and N68335-11-C-0425 awarded by theDepartment of the Defense (Navy). The government has certain rights inthis invention.

TECHNICAL FIELD OF THE INVENTION

This invention relates, in general, to electrolytes that provide for ionmovement and, in particular, to electrolytes that provide for ionmovement in electrochemical energy storage devices, such as batteries,supercapacitors, and similar devices.

BACKGROUND OF THE INVENTION

Without limiting the scope of the present invention, the background willbe described in relation to rechargeable lithium-ion batteries, as anexample. Rechargeable lithium-ion batteries have improved safety andhigh performance characteristics such as high capacities, high rates,long cycle lives, and wide temperature ranges. These characteristics areneeded for a variety of applications such as electric vehicles,aircraft, and consumer electronics, for example. The electrolytes usedfor current Li-ion batteries, however, are highly flammable and thisplay a key role in battery safety. Too often, flammability is reduced atthe expense of performance by the use of non-flammable compounds thatnegatively impact performance. The development of a non-flammableelectrolyte that does not reduce battery performance is needed.

SUMMARY OF THE INVENTION

It would be advantageous to improve flammability profiles inelectrolytes that provide for ion movement in electrochemical energystorage devices, such as batteries, supercapacitors, and similardevices. It would also be advantageous to provide the aforementionedsignificant improvements to the safety of the electrochemical energystorage devices while maintaining high performance. It would also bedesirable to enable a chemical-based solution that provides both highperformance and improved safety such that the electrolyte offersadequate ionic conductivity over the desired operating temperaturerange, a wide electrochemical stability window, high capacities for boththe cathode and anode, low electrode-electrolyte interfacial resistance,and reduced flammability. The electrolyte composition is a compromisebetween these various factors and the specific composition of theelectrolyte will depend on the application and its requirements, such asdesired temperature range, electrode materials, and rate, for example.

To better address one or more of these concerns, an electrolyte thatprovides for ion movement in electrochemical energy storage devices isdisclosed. In one embodiment, the electrolyte includes a lithium saltfrom about 3% to about 20% by weight, a primary solvent from about 15%to about 25% by weight, wide-temperature co-solvents from about 14% toabout 55% by weight, interface forming compounds from about 0.5% toabout 2.0% by weight, and a flame retardant compound from about 6% toabout 60% by weight. The electrolyte interacts with the positive andnegative electrodes of the electrochemical storage device to provideboth high performance and improved safety such that the electrolyteoffers adequate ionic conductivity over the desired operatingtemperature range, a wide electrochemical stability window, highcapacities for both the cathode and anode, low electrode-electrolyteinterfacial resistance, and reduced flammability. These and otheraspects of the invention will be apparent from and elucidated withreference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawings will be provided by the Office upon request and paymentof the necessary fee.

For a more complete understanding of the features and advantages of thepresent invention, reference is now made to the detailed description ofthe invention along with the accompanying figures in which correspondingnumerals in the different figures refer to corresponding parts and inwhich:

FIG. 1A is a portion of a chemical glossary that describes structures ofselected electrolyte components grouped as classes of compounds andspecifically lithium salts in FIG. 1A;

FIG. 1B is a portion of a chemical glossary that describes structures ofselected electrolyte components grouped as classes of compounds andspecifically solvents in FIG. 1B;

FIG. 1C is a portion of a chemical glossary that describes structures ofselected electrolyte components grouped as classes of compounds andspecifically flame retardant additives in FIG. 1C;

FIG. 1D is a portion of a chemical glossary that describes structures ofselected electrolyte components grouped as classes of compounds andspecifically solid electrolyte interface-forming compounds in FIG. 1D;

FIG. 2 is a graph of mass normalized self-extinguishing time versus massamount (wt %) of flame retardant additive FRA-D within the electrolytemixture;

FIG. 3 is a graph showing the ionic conductivity of electrolytes as afunction of temperature;

FIG. 4 is a graph showing the electrochemical impedance spectra of fullcells containing electrolytes with different amounts of flame retardantadditive (FRA-D) and the comparison with the baseline electrolyte;

FIG. 5 is a graph showing the voltage profile of first formation chargecycle for full cell coin cells comparing various electrolytes;

FIG. 6 is a graph showing the voltage profile of first formation chargecycle for additional full cell coin cells with various electrolytes;

FIG. 7 is a graph showing the voltage profile of the first formationdischarge cycle for full cell coin cells with various electrolytes;

FIG. 8 is a graph showing the cycle life testing for 100 cycles (afterthree formation cycles for full cells with electrolytes CE-C1 (blue),F-006-B-01 (green), and F-007-D-01 (orange), and after five formationcycles for remaining cells) for full cell coin cells, wherein cells werecycled from 3.0V to 4.1V at 1.0 C rate at room temperature;

FIG. 9 is a graph showing the cycle life testing for 100 cycles (afterthree formation cycles at RT) for full cell coin cells using variouselectrolytes comparing baseline with WTA (blue & indigo), baseline withWTA and 20% cyclic phosphaze (FRA-D) (green), baseline with WTA and 20wt % TMP (FRA-C) (orange), baseline with WTA and 20 wt % DMMP (FRA-B)(violet), and baseline with WTA and 20 wt % TTFEP (FRA-A) (red), whereinthe cells were cycled from 3.0V to 4.1V at 1.0 C rate at roomtemperature;

FIG. 10 is a graph showing the voltage profile of 1st discharge cycle at−40° C. (after 5 formation cycles at RT—6^(th) overall discharge) forfull cell coin cells comparing the commercial baseline electrolyte(blue), base composition A electrolyte with no FRA-D added (green) andwith 10% FRA-D added (orange), and base composition B electrolyte withno FRA-D added (violet), with 10% FRA-D added (red), with 15% FRA-Dadded (black), and with 20% FRA-D added (pink), wherein the cells werecycled from 3.0V to 4.1V at C/10 rate with the charge step occurring at25° C. and the discharge step at −40° C.;

FIG. 11 is a graph showing the specific capacity of 30 discharge cyclesat +71° C. (after 5 formation cycles at room temperature) for full cellcoin cells with various electrolytes;

FIG. 12 is a graph showing the specific capacity of 30 discharge cyclesat +71° C. (after 5 formation cycles at RT—cycles 6-35 overall for CE-Cand F-007-D cells and post-low temperature cycling—cycles 9-38 overallfor F-032-D, F-037-D, F-038-D, and F-033-D cells) for full cell coincells comparing the commercial baseline electrolyte (blue), compositionA electrolyte and composition B electrolytes with various amounts ofWTA-A;

FIG. 13 is a graph showing the specific capacity of 30 discharge cyclesat +71° C. (after 5 formation cycles at RT)—cycles 6-35 overall for CE-Cand F-007-D cells and post-low temperature cycling—cycles 9-38 overallfor F-032-D, F-040-D, and F-041 cells) for full cell coin cells withdifferent electrolytes compared with a baseline electrolyte (blue);

FIG. 14 is a graph showing the electrochemical impedance spectra of fullcells composed comparing various cells containing different amounts ofwide temperature additive (WTA-A); and

FIG. 15 is a graph showing the voltage profile of 1st discharge cycle at−40° C. (after five formation cycles at RT—6th overall discharge) forfull cell coin cells with varying amounts of WTA at 0 wt % (orange), 0.5wt % (violet), 1.0 wt % (red) and 2.0 wt % (black).

DETAILED DESCRIPTION OF THE INVENTION

While the making and using of various embodiments of the presentinvention are discussed in detail below, it should be appreciated thatthe present invention provides many applicable inventive concepts, whichcan be embodied in a wide variety of specific contexts. The specificembodiments discussed herein are merely illustrative of specific ways tomake and use the invention, and do not delimit the scope of the presentinvention.

Referring initially to FIG. 1A through FIG. 1D, therein is depicted,collectively, a chemical glossary relative to an electrolyte for anelectrochemical storage device presented herein. The electrolyteinteracts with at least one positive electrode configured to store andrelease lithium-ions and at least one negative electrode configured tostore and release lithium-ions. The at least one positive electrode andthe at least one negative electrode form portions of the electrochemicalstorage device, which may be a battery, lithium-ion battery, orsupercapacitor for example.

The electrolyte is based on specific combinations of compounds thatresult in an electrolyte with reduced flammability and wide temperaturerange operation. By using appropriately designed mixtures of specificcompounds with specific amounts of the components, electrolytes withboth improved safety and high performance may be obtained. This uniquecombination of electrolyte properties may be obtained with specificcomponents that when combined in specific amounts impart the desiredelectrolyte properties. The desired combination of electrolyteproperties is not obtained with prior electrolyte compositions that showeither reduced flammability or wide temperature operation, but not bothreduced flammability and wide temperature operation.

Non-aqueous liquid electrolytes are typically composed of a salt, one ormore solvents, and other additives. In order to obtain an electrolytewith the desired properties, specific types of salts, solvents, andadditives are required to be combined in specific amounts.

For a lithium-ion conducting electrolyte, lithium salts such as lithiumhexafluorophosphate (LiPF₆), lithium perchlorate (LiClO₄), lithiumbistrifluoromethanesulfonimide (LiTFSI), lithium tetrafluoroborate(LiBF₄), or other salts are suitable. The electrolyte's salt affects theionic conductivity based on the ion-ion and ion-solvent interactions ofthe salt and solvents. The concentration of the salt is important tooptimize the ionic conductivity. Generally high concentrations of ionsincrease the ionic conductivity up to a point, after which theconductivity decreases. Additionally, the solubility of the salt and thesolvents is important. Electrolytes can be made for with a variety ofsalts or mixtures of salts. Lithium salt concentrations of about 0.8 toabout 1.2 M LiPF₆ were used to obtain an electrolyte with high ionicconductivity, but the use of other salts is not precluded.

The type and amount of solvents and co-solvents is critically importantto the electrolyte properties. Ethylene carbonate (EC) provides primarycoordination of Li-ions in the first (primary) solvation sphere.However, Ethylene carbonate (EC) is a solid at room temperature(freezing point of +38° C.), and therefore by itself is not an adequatesolvent for an electrolyte that functions as liquid at room temperature.Generally, one or more solvents are used in addition to ethylenecarbonate for typical lithium-ion battery electrolytes. The solventstypically combined with ethylene carbonate include dimethyl carbonate(DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). Thetype and amount of the solvents can affect a number of importantproperties including the temperature range (both freezing/melting pointand boiling point) and interfacial properties. The melting points andboiling points of electrolyte solvents are presented in Table 1. Ethylmethyl carbonate has a lower freezing point (−55° C.) compared with DEC(−43° C.) or DMC (3° C.). Therefore for improved low temperature rangeoperation, EMC is a preferred solvent based on its solubility with ECand its wide temperature range (freezing point of −55° C. and boilingpoint of +108° C.). Although propylene carbonate (PC) has a widetemperature range (melting point of −55° C.; boiling point of +240° C.),the use of PC is problematic since it does not allow reversibleintercalation of Li-ions within graphitic carbons. By combining EMC withEC at appropriate ratios, an electrolyte with good low temperatureperformance and good ionic conductivity can be obtained. Too high aconcentration of EC will reduce the low temperature performance sincethe melting point of EC is above room temperature. In one embodiment, adesirable mass ratio of the EC in the electrolyte solution is in therange of 10-25 wt % and a desirable mass ratio of the EMC in theelectrolyte solution is 15-55 wt %, depending on the composition of theadditional electrolyte components.

TABLE 1 Melting points and freezing points of electrolyte solvents.Melting point, Boiling point, Compound T_(mp) ° C. T_(bp) ° C.Acetonitrile −46 82 Propylene carbonate −55 240 Methyl acetate −98 571,3-Dioxolane −95 78 Dibutyl ether −98 143 Triethylamine −115 891-Methylpyrrolidine −90 78 1,2-Dimethoxyethane −58 85 Ethylene carbonate38 243 Dimethyl carbonate 3 91 Diethyl carbonate −43 125 Ethyl methyl−55 108 carbonate Ethyl acetate −84 77 Ethyl propionate −73 99 Ethylbutyrate −93 120 Dimethyl −50 181 methylphosphonate

Flame retardant compounds can be added to the electrolyte to reduce theflammability. The type and amount of the electrolyte is criticallyimportant in order to obtain the desired properties. The flame retardantcompound must be soluble in the electrolyte. Additionally, the flameretardant compound should not significantly reduce the ionicconductivity in order to attain high performance. Another importantfactor is the electrochemical stability of the flame retardant compound.The flame retardant compound can negatively affect performance if it isreduced or oxidized at anodic or cathodic electrode respectively. It isdesirable to have a flame retardant compound that has a reductionpotential higher than that of other solvents in the mixture such asethylene carbonate. If the flame retardant compound has a reductionpotential higher than other solvents in the mixture, the interfaciallayer between the anode and the electrolyte will be predominately formedby the other solvents rather than the flame retardant compound. Sinceother solvents such as ethylene carbonate form conductive and protectivesolid-electrolyte interface layers, this is a desirable outcome.

Specifically the use of cyclic polyphosphazene derivatives which includeeither fluorinated groups, methoxy groups, ethoxy groups, ethers, linearaliphatics, and chlorinated derivatives are desirable as flame retardantcompounds. These must be combined in appropriate amounts within theelectrolyte in order to impart reduced flammability while maintainingdesirable properties including ionic conductivity and wide operatingtemperature. Using a concentration of between 10-50% of2-ethoxy-2,4,4,6,6-pentafluoro-1,3,5,2λ⁵,4λ⁵,6λ⁵ triazatriphosphinine, acyclic polyphosphaze derivative, imparts reduced flammability whilemaintaining reasonable electrolyte properties. Substantially higherconcentrations of this compound were not miscible in an electrolytemixture composed of LiPF₆, EC, EMC, and methyl butyrate (MB).

Obtaining good operation at low temperatures (e.g., −20° C. and below)for the electrolyte requires adding specific compounds at specificamounts. The addition of compounds which are soluble in the electrolytemixtures and which have lower freezing points than EMC provides a methodto lower the operating temperature of the electrolyte. The use ofcompounds such as methyl butyrate, ethyl propionate, ethyl butyrate,1,3-dioxolane, and ethyl acetate in the electrolyte at concentrationranges of about 10% to about 50% provides improved low temperatureoperation.

The proper type and amount of the salt, primary solvents, solvents forlow temperature operation, and flame retardant additives does not initself provide a sufficient combination of properties to also enablehigh temperature stability without the addition of a specific amount ofa solid electrolyte interface forming compound. Ethylene carbonate (EC)readily undergoes reductive decomposition on the surface of the anodic(negative) electrode, and it forms a solid electrolyte interface (SEI)layer. Additives such as vinylene carbonate (VC), vinylethylenecarbonate (VEC), and phenylethylene carbonate (PhEC) have a higherreduction potential compared to EC. These additives will bepreferentially reduced at the anode and therefore the interface layerwill be formed predominantly from these compounds. These additives canimprove the high temperature stability of the cells, since hightemperature electrolyte decomposition is controlled by a stable SEIlayer.

Specific types and amounts of the additives are needed in order tobalance the low temperature and high temperature performance. Too muchof these additives will result in a highly resistive layer that willreduce the low temperature performance since the concentration of theinterface forming compound affects the interfacial charge transferresistance. At low temperatures, cell resistance will increase since theresistance of the electrolyte and the interface increase at lowtemperatures based on the temperature dependence of ion conduction. Anadequate amount of an interface forming compound is needed in order toprovide high temperature stability. By using vinylene carbonate (VC) atamounts of between about 0.5 wt % and about 2.0 wt % results in balanceof good low temperature performance and high temperature stability. Apreferred amount of VC is about 1.0 wt %.

Some of the compounds used to improve the performance also affect theflammability. In particular the flammability of ethylene carbonate andvinylene carbonate are relatively high compared with other solvents.Therefore reducing the amount of these compounds can decrease theflammability. Replacing ethyl methyl carbonate with other compounds suchas methyl butyrate, ethyl propionate, and butyl propionate can reducethe flammability. While individually many of these compounds (e.g., MB,VC, and cyclic phosphaze derivatives) have been shown to be effective ateither improving wide temperature operation or reducing flammability,the combination of these compounds at appropriate amounts is required inorder to impart the desired properties of reduced flammability andhigh-performance.

In one embodiment, the electrolyte may include a lithium salt from about3% to about 20% (or to about 25% or to about 30%) by weight, a primarysolvent from about 15% to about 25% by weight, wide-temperatureco-solvents from about 14% to about 55% by weight, and a flame retardantcompound from about 6% to about 60% by weight. In one implementation, aninterface forming compounds from about 0.5% to about 2.0% by weight mayalso be included. A suitable lithium salt may be a salt selected fromthe group consisting of lithium hexafluorophosphate, lithiumbistrifluoromethanesulfonimide (LiTFSI), lithium tetrafluoroborate(LiBF4), and combinations thereof. In one embodiment, the lithium saltmay preferably be lithium hexafluorophosphate. The primary solvent maybe a solvent selected from the group of solvents consisting of ethylenecarbonate, diethyl carbonate, dimethyl carbonate, ethyl methylcarbonate, ethyl propionate, methyl butyrate, ethyl acetate, ethylbutyrate and/or 1,3-dioxolane, and mixtures thereof. In one embodiment,the primary solvent may preferably be ethylene carbonate. Thewide-temperature co-solvent may be a solvent selected from the groupconsisting of ethylene carbonate, diethyl carbonate, dimethyl carbonate,ethyl methyl carbonate, ethyl propionate, methyl butyrate, ethylacetate, ethyl butyrate and/or 1,3-dioxolane, and mixtures thereof. Inone embodiment, the wide-temperature co-solvents may preferably be ethylmethyl carbonate.

A suitable wide-temperature co-solvent may be methyl butyrate or amixture of ethyl methyl carbonate and methyl butyrate. The interfaceforming compound may be a compound selected from the groups consistingof vinylene carbonate, vinyl ethylene carbonate, lithiumbis(oxalato)borate, and combinations thereof. The interface formingcompound may preferably be vinylene carbonate. The flame retardantcompound may be a compound selected from the group consisting of cyclicphosphonates, phosphonates, phosphates, ionic liquids, and combinationsthereof. The flame retardant compound may preferably be2-ethoxy-2,4,4,6,6-pentafluoro-1,3,5,2λ⁵,4λ⁵,6λ⁵ triazatriphosphinine.

In another embodiment, the electrolyte include a lithium salt from about9% to about 16% by weight; a primary solvent from about 16% to about 25%by weight; wide-temperature co-solvents from about 47% to about 53% byweight; and a flame retardant compound from about 12% to about 18% byweight. In another embodiment, the electrolyte includes a lithium saltfrom about 10% to about 16% by weight, the lithium salt includinghexafluorophosphate; a primary solvent from about 16% to about 22% byweight, the primary solvent including ethylene carbonate;wide-temperature co-solvents from about 49% to about 52% by weight, thewide-temperature co-solvents including ethyl methyl carbonate and methylbutyrate; interface forming compounds from about 0.5% to about 2.0% byweight, the interface forming compounds including vinylene carbonate;and a flame retardant compound from about 10% to about 18% by weight,the flame retardant compound including2-ethoxy-2,4,4,6,6-pentafluoro-1,3,5,2λ⁵,4λ⁵,6λ⁵ triazatriphosphinine.In a still further embodiment, the electrolyte includes a lithium saltfrom about 9% to about 15% by weight, the lithium salt includinghexafluorophosphate; a primary solvent from about 19% to about 25% byweight, the primary solvent including ethylene carbonate;wide-temperature co-solvents from about 47% to about 53% by weight, thewide-temperature co-solvents including ethyl methyl carbonate; interfaceforming compounds from about 0.5% to about 2.0% by weight, the interfaceforming compounds including vinylene carbonate; and a flame retardantcompound from about 10% to about 18% by weight, the flame retardantcompound including 2-ethoxy-2,4,4,6,6-pentafluoro-1,3,5,2λ⁵,4λ⁵,6λ⁵triazatriphosphinine.

With respect to the last two formulations presented herein above, aspecific example of the electrolyte may be a combination of about 12.9%by weight lithium hexafluorophosphate, about 18.7% by weight ethylenecarbonate, about 14.3% by weight ethyl methyl carbonate, about 38.1% byweight methyl butyrate, about 1.0% by weight vinylene carbonate, and15.0% by weigth 2-ethoxy-2,4,4,6,6-pentafluoro-1,3,5,2λ⁵,4λ⁵,6λ⁵triazatriphosphinine. Another specific example may be a combination ofabout 12.1% by weight lithium hexafluorophosphate, about 21.6% by weightethylene carbonate, about 50.4% by weight ethyl methyl carbonate, 1.0%by weight vinylene carbonate, and 15.0% by weight2-ethoxy-2,4,4,6,6-pentafluoro-1,3,5,2λ⁵,4λ⁵,6λ⁵ triazatriphosphinine.

The present invention will now be illustrated by reference to thefollowing non-limiting working examples wherein procedures and materialsare solely representative of those which can be employed, and are notexhaustive of those available and operative. Examples I-VI and theaccompanying test methods illustrate the advantages of an electrolytebased on specific combinations of compounds that result in anelectrolyte with reduced flammability and wide temperature rangeoperation.

Example I

Electrolytes were prepared in an inert atmosphere (Argon) glovebox bycombining salts, solvents, co-solvents, and additives. Compounds wereobtained from chemical manufacturers or suppliers. Specific compositionsprepared are shown in Table 2. The structures of selected electrolytecomponents grouped as classes of compounds are shown in Table 2. Thestructures of selected electrolyte components grouped as classes ofcompounds are shown in FIGS. 1A, 1B, 1C, and 1D. The compound names andtheir abbreviations are listed as follows:

Salts

Lithium hexafluorophosphate (LiPF₆)

Lithium bistrifluoromethanesulfonimide (LiTFSI)

Lithium tetrafluoroborate (LiBF₄)

Solvents/Co-Solvents

Ethylene carbonate (EC)

Diethyl carbonate (DEC)

Dimethyl carbonate (DMC)

Ethyl methyl carbonate (EMC)

Ethyl Propionate (EP)

Methyl Butyrate (MB)

Flame Retardant Compounds

Tris(2,2,2-trifluoroethyl)phosphate (TTFEP) (FRA-A)

Dimethyl methylphosphonate (DMMP) (FRA-B)

Trimethyl phosphate (TMP) (FRA-C)

2-Ethoxy-2,4,4,6,6-pentafluoro-1,3,5,2λ⁵,4λ⁵,6λ⁵ triazatriphosphinine(Phoslyte—Hishicolin E, CP1, FRA-D)

1-ethyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide (EMI-TFSI) (FRA-E)

2,4,6-Tris(trifluoromethyl)-1,3,5-triazine (TTFMT)

Solid Electrolyte Interface Formers/Wide Temperature Additives

Vinylene carbonate (VC) (WTA-A)

Lithium bis(oxalato)borate (LiBOB)

Dimethyl acetamide (DMAC)

Component amounts were measured by mass and are shown by percent byweight, i.e., weight percent (wt. %), within the combined electrolyte.For the salt, the molarity was also calculated. The use of glass vialswas avoided due to risk of contamination due to etching from freehydrogen fluoride; therefore, electrolytes were prepared and stored in99.5% pure aluminum bottles obtained from Elemental Container. Thenotation used for the electrolytes consists of a base letter (F in thiscase) followed by a three digit number to identify the composition(e.g., 001). Then a group identifier is included (Group A: ControlElectrolyte; Group B: Electrolyte that included a wide temperatureadditive (WTA); Group C: Electrolyte that includes a flame retardantadditive (FRA); and Group D: Electrolyte that includes a WTA and a FRA).Following this, a specific batch number is included (e.g., ‘01’, ‘02’).

For comparison, baseline electrolytes obtained from commercial vendors(e.g., Novoltye, MTI, or BASF). These baseline electrolytes were 1MLiPF₆ in EC:DEC (1:1 v/v) (Novolyte) (notated as control electrolyte-A“CE-A”) and 1M LiPF₆ in EC+DMC+DEC (1:1:1 vol ratio) (MTI) (notated as“CE-B”), and 1.2 M LiPF₆ in EC:EMC (3:7 et %) (notated as “CE-C”).

To prevent exposure to moisture which will degrade the electrolyte, allelectrolytes were stored in an inert atmosphere (Arbon) glovebox. Waterlevels in the electrolyte were determined using Karl-Fisher titrationfor selected samples.

TABLE 2 Electrolyte compositions describing the mass ratio of each ofthe components. Primary Flame Retardant Additive Salt Solvents WideTemperature Additives (WTA) (FRA) Salt Salt Molarity EC EMC MB EP VCLiBOB DMAC FRA FRA FRA Notation Comp. (wt %) (M) (wt %) (wt %) (wt %)(wt %) (wt %) (wt %) (wt %) (wt %) Comp. Notation F-001-A-XX LiPF₆ 11.51.0 50.1 38.3 F-002-B-XX LiPF₆ 12.6 1.0 28.5 28.5 28.5 2.0 F-003-D-XXLiPF₆ 11.3 1.0 25.6 25.6 25.6 1.8 10.0 CP1 FRA-D F-004-B-XX LiPF₆ 12.61.0 21.5 49.4 14.5 2.0 F-005-D-XX LiPF₆ 11.3 1.0 19.4 44.4 13.0 1.8 10.0CP1 FRA-D F-006-B-XX LiPF₆ 14.8 1.2 21.0 48.1 14.1 2.0 F-007-D-XX LiPF₆13.3 1.2 18.9 43.3 12.7 1.8 10.0 CP1 FRA-D F-008-D-XX LiPF₆ 11.8 1.216.8 38.5 11.3 1.6 20.0 CP1 FRA-D F-009-D-XX LiPF₆ 11.8 1.2 16.8 38.511.3 1.6 20.0 TMP FRA-C F-010-D-XX LiPF₆ 11.8 1.2 16.8 38.5 11.3 1.620.0 DMMP FRA-B F-011-D-XX LiPF₆ 11.8 1.2 16.8 38.5 11.3 1.6 20.0 TTFEPFRA-A F-012-D-XX LiPF₆ 11.8 1.2 16.8 38.5 11.3 1.6 20.0 TTFMT FRA-FF-013-D-XX LiPF₆ 13.0 1.2 21.9 51.1 2.0 2.0 10.0 CP1 FRA-D F-014-D-XXLiPF₆ 14.3 1.2 24.2 56.5 2.0 2.0 1.0 F-015-D-XX LiPF₆ 12.8 1.2 21.7 50.52.0 2.0 1.0 10.0 CP1 FRA-D F-016-D-XX LiPF₆ 13.0 1.2 18.5 42.5 12.4 1.82.0 9.8 CP1 FRA-D F-017-D-XX LiPF₆ 12.9 1.2 18.3 42.0 12.3 1.8 2.0 1.09.7 CP1 FRA-D F-018-D-XX LiPF₆ 10.9 1.2 15.8 48.2 10.7 1.7 2.0 1.0 9.7CP1 FRA-D F-019-D-XX LiPF₆ 13.1 1.2 22.2 51.7 2.0 1.0 10.0 CP1 FRA-DF-023-D-XX LiPF₆ 11.6 1.2 19.6 45.8 2.0 1.0 20.0 CP1 FRA-D F-021-D-XXLiPF₆ 13.1 1.2 18.7 45.9 12.6 1.8 1.0 9.9 CP1 FRA-D F-022-D-XX LiPF₆11.7 1.2 16.6 38.1 11.2 1.6 1.0 19.8 CP1 FRA-D F-023-D-XX LiPF₆ 13.3 1.222.4 52.3 2.0 10.0 CP1 FRA-D F-024-D-XX LiPF₆ 11.7 1.2 19.9 46.4 2.020.0 CP1 FRA-D F-025-D-XX LiPF₆ 13.0 1.2 18.9 43.3 12.8 2.0 10.0 CP1FRA-D F-026-D-XX LiPF₆ 11.5 1.2 16.7 38.4 11.4 2.0 20.0 CP1 FRA-DF-027-B-XX LiPF₆ 14.8 1.2 25.0 58.2 2.0 F-028-C-XX LiPF₆ 13.6 1.2 22.953.5 10.0 CP1 FRA-D F-029-C-XX LiPF₆ 12.1 1.2 20.4 47.5 20.0 CP1 FRA-DF-030-B-XX LiPF₆ 15.4 1.2 22.3 16.9 45.4 F-031-B-XX LiPF₆ 15.1 1.2 21.816.6 44.5 2.0 F-032-D-XX LiPF₆ 13.8 1.2 20.0 15.3 40.9 10.0 CP1 FRA-DF-033-D-XX LiPF₆ 13.5 1.2 19.6 14.9 40.0 2.0 10.0 CP1 FRA-D F-034-B-XXLiPF₆ 15.3 1.2 22.1 16.9 45.2 0.5 F-035-B-XX LiPF₆ 15.2 1.2 22.0 16.845.0 1.0 F-036-B-XX LiPF₆ 15.1 1.2 21.9 16.7 44.7 1.6 F-037-D-XX LiPF₆13.8 1.2 19.9 15.2 40.6 0.5 10.0 CP1 FRA-D F-038-D-XX LiPF₆ 13.7 1.219.8 15.1 40.4 1.0 10.0 CP1 FRA-D F-039-D-XX LiPF₆ 14.6 1.2 21.2 16.143.1 5.0 CP1 FRA-D F-040-D-XX LiPF₆ 13.1 1.2 18.9 14.4 38.6 15.0 CP1FRA-D F-041-D-XX LiPF₆ 12.3 1.2 17.9 13.5 36.3 20.0 CP1 FRA-D F-042-D-XXLiPF₆ 14.5 1.2 21.1 16.0 42.9 0.5 5.0 CP1 FRA-D F-043-D-XX LiPF₆ 13.01.2 18.8 14.3 38.4 0.5 15.0 CP1 FRA-D F-044-D-XX LiPF₆ 12.2 1.2 17.713.5 36.1 0.5 20.0 CP1 FRA-D F-045-D-XX LiPF₆ 12.9 1.2 18.7 14.3 38.11.0 15.0 CP1 FRA-D F-046-D-XX LiPF₆ 12.1 1.2 21.6 50.4 1.0 15.0 CP1FRA-D F-052-D-XX LiPF₆ 11.6 1.2 16.6 12.7 34.1 25.0 CP1 FRA-D F-053-D-XXLiPF₆ 7.7 1.2 11.1 8.5 22.7 50.0 CP1 FRA-D F-054-D-XX LiPF₆ 6.2 1.2 8.96.7 18.2 60.0 CP1 FRA-D F-055-D-XX LiPF₆ 3.9 1.2 5.5 4.2 11.4 75.0 CP1FRA-D F-057-B-XX LiPF₆ 14.3 1.2 25.4 59.4 1.0 F-058-D-XX LiPF₆ 7.1 1.212.6 29.4 1.0 50.0 CP1 FRA-D F-059-D-XX LiPF₆ 5.6 1.2 10.0 23.4 1.0 60.0CP1 FRA-D F-060-D-XX LiPF₆ 3.5 1.2 6.2 14.4 1.0 75.0 CP1 FRA-D

Example II

Electrolyte flammability was measured using multiple methods including“wick tests”, flash point tests, and oxygen calorimetry tests. For thewick tests, a ˜1.5 inch piece of glass fiber tape saturated with 1 mL ofthe electrolyte. The saturated glass fiber tape was suspended above analcohol burner (using denatured ethanol as the fuel) to a position inwhich the tip of the flame contacted the mid-point of the glass fibertape. The flame height and burn time were recorded. Two to five trialswere performed for each electrolyte, and trials were recorded withvideo. Averages of initial burn time and initial flame height werecalculated. The mass normalized self-extinguishing time was determinedfrom the mass and burn time. To determine flash points of electrolytes,samples were tested by Galbraith Laboratories using ASTM D56 and D93methods. For electrolytes with flash points expected to be below 80° C.,the ASTM D56 (Tag Closed Cup) test was performed. For electrolytes withflash points expected to be above 80° C., the ASTM D93 (Closed Cup) testwas performed. Selected samples were also sent to Southwest ResearchInstitute (SwRI) for oxygen consumption calorimetry tests according toASTM E 1354-1 (Standard Test Method for Heat and Visible Smoke ReleaseRates for Materials and Products Using an Oxygen Consumptioncalorimeter) using a at a heat flux of 15 kW/m².

The flammability properties of candidate electrolytes and a baselineelectrolyte are shown in Table 3. The data shows that the candidateelectrolytes show significantly lower flammability compared with thebaseline electrolyte composition.

TABLE 3 Comparison of flammability of candidate electrolytes fromtesting performed. Mass Flame Normalized Time to Retardant Self-ignition Additive Extin- from (FRA) guishing ASTEM Flash Candidate FRA %Time E 1354-10 point Category Electrolyte ID FRA (sec/g) (sec) (° C.)Baseline CE-C —  0 19.4 7 26 A F-006-B —  0 20.8 8 N/A F-007-D D 10 1.714 N/A F-008-D D 20 0.5 26 N/A B F-030-B —  0 18.4 N/A 16 F-039-D D  523.7 N/A N/A F-032-D D 10 2.3 13.3 24 F-040-D D 15 2.0 25.7 >100 F-041-DD 20 1.2 N/A N/A

Each of the three different flammability tests performed shows similartrends. The conventional electrolyte (CE-C) easily caught fire. Howeverthe flammability was decreased by increasing the amounts of flameretardant additive.

The flashpoints of selected electrolytes are shown in Table 4. Thecompositions of these electrolytes are described in Example I. The flashpoint is the lowest temperature at which a liquid will form an explosivevapor under normal atmospheric conditions. It is an importantmeasurement of volatility and allows appropriate safety measures to betaken when transporting, storing or handling flammable liquids. Liquidsare classified on the basis of flash point measurement: extremelyflammable, having a flash point lower than 0° C. and a boiling pointless than 35° C.; highly flammable, having a flash point below 21° C.;and flammable, having a flash point between 21° C. and 55° C. A liquidwith a flash point in the range 32° C.-55° C. may not require as manyprecautions in use as it will not form an explosive atmosphere unless itis stored at elevated temperature. A flash point >93° C. is not acombustible liquid. With electrolyte composition F-040-D (with 15 wt %FRA-D), the flashpoint increased to >100° C. and therefore this would beconsidered a non-combustible liquid based on the flash point tests.

TABLE 4 Results of the flashpoint tests for various electrolytes.Electrolyte ID % FRA Testing Flash Point (° C.) CE-C2 0 ASTM D56-05 26F-030-B 0 ASTM D56-05 16 F-032-D 10 ASTM D93-11 24 F-040-D 15 ASTMD93-11 >100

Different electrolytes with the same amount of flame retardant additiveresulted in different flammabilities as can be observed from the data inTable 3. Therefore, the flammability is affected by additionalcomponents in the electrolyte in addition to the flame retardantadditive. The flammabilities of solvents used in the electrolyte wereevaluated using the wick test method and the data is presented in [0057]Table 5. This data supports that different solvents can increase ordecrease the flammability. In particular, the mass normalized selfextinguishing times of ethylene carbonate and vinylene carbonate arerelatively higher than other solvents. Therefore reducing the amount ofethylene carbonate and vinylene carbonate can decrease the flammability.In addition, reducing or replacing ethyl methyl carbonate with othercompounds such as methyl butyrate, ethyl propionate, and butylpropionate can reduce the flammability. The testing also supported thatthe flame retardant additives were indeed non-flammable.

TABLE 5 Flammability test results for electrolyte components andadditives comparing average maximum flame heights and mass normalizedself-extinguishing times. Average Mass Normalized Candidate ElectrolyteMaximum Self-Extinguishing Component/Additive Flame Height (in) Time(sec/g) Ethylene Carbonate (EC) 5.6 30.8 Diethyl Carbonate (DEC) 9 17.9Dimethyl Carbonate (DMC) 7.5 15.9 Ethyl Methyl Carbonate (EMC) 8.9 15.5Vinylene Carbonate (VC) 7.2 23.2 Ethyl Propionate (EP) 12.2 12.9 MethylPropionate (MP) 9.8 11.5 Methyl Butyrate (MB) 11.5 13.22,4,6-Tris(trifluoromethyl)- 0 0 1,3,5-triazine (TTFMT) Hishicol in E(HE) 0 0

Different flame retardant additives can be used to reduce theflammability of the electrolyte. The effectiveness of different flameretardant additives (FRAs) in a specific formulation was evaluated.Electrolytes prepared with 20 wt % of various FRAs are compared in Table6. From this data, 2-ethoxy-2,4,4,6,6-pentafluoro-1,3,5,2λ⁵,4λ⁵,6λ⁵triazatriphosphinine, the cyclic phosphazene derivative (Hishicolin E,FRA-D), showed the most significant reduction in the flammability of theelectrolyte.

TABLE 6 Comparison of a baseline electrolyte with no flame retardantadditive (FRA) against electrolytes that contained 20 wt % FRA inaddition to the baseline electrolyte. Mass Normalized Flame RetardantSelf- Candidate Additive (FRA) Extinguishing Electrolyte FRA ID % FRATime (sec/g) F-006-B-01 — 0 20.8 F-011-D-01 TTFEP 20 16.9 F-010-D-01DMMP 20 13.5 F-009-D-01 TMP 20 13.2 F-008-D-01 FRA-D 20 0.5 F-012-D-01EMI-TFSI 20 21.7

To determine the amount of FRA-D needed to reduce the flammability, theFRA-D wt % was varied and the electrolyte composition was kept constant.Compositions of electrolytes prepared are shown in Table 7.

TABLE 7 Electrolyte compositions with solvents/additives correspondingto relative wt %, and flame retardant additive relating to wt %. SaltSolvents/Additives Molarity (Relative-wt %) FRA-D Notation Composition(M) EC (%) EMC (%) (wt %) NFE-O-A01 LiPF₆ 1.2 3 7 2 NFE-O-A02 LiPF₆ 1.23 7 4 NFE-O-A03 LiPF₆ 1.2 3 7 6 NFE-O-A04 LiPF₆ 1.2 3 7 8 NFE-O-A05LiPF₆ 1.2 3 7 10 NFE-O-A06 LiPF₆ 1.2 3 7 20

In Table 8, corresponding mass normalized self extinguishing time andaverage maximum flame height are shown from the results of three wicktests. Flammability categories were designated according to individualself-extinguishing time. The criteria used for designating theelectrolyte as nonflammable flame retardant or flammable were based onthe mass-normalized self extinguishing time (SET): <1 sec/g:non-flammable; 1-10 sec/g: flame retardant; >10 sec/g flammable. Shownin FIG. 2 is a graph of the wt % of the NFA and the self extinguishingtime. The data shows that for this electrolyte composition, theelectrolyte becomes flame retardant at a wt % of between about 8% toabout 10% and becomes non-flammable from this test and criteria at acomposition of >10 wt %. Therefore of composition of about 10 wt % andpreferably higher is needed to impart flame retardant properties to theelectrolyte.

TABLE 8 Flammability testing of electrolytes with different amounts ofFRA-D. Average Mass Maximum Flame Normalized Height During Self FlameExposure Extinguishing Flammability Electrolyte (in) Time (sec/g)Category NFE-O-A01 7 20.1 Flammable NFE-O-A02 7 18.1 Flammable NFE-O-A037 17.8 Flammable NFE-O-A04 3 11.3 Flammable NFE-O-A05 2.5 1.4 FlameRetardant NFE-O-A06 0.3 0.3 Non-Flammable

Example III

The ionic conductivity for the electrolytes was evaluated in order todetermine the performance of the electrolyte over wide temperatureranges. To measure conductivity, a Mettler-Toledo InLab 710 conductivitycell with an in-house fabricated air-tight holder was used. The cell wasassembled in an inert atmosphere glovebox and then sealed completelybefore removal for measurements. The air-tight holder for theconductivity was fabricated to provide an air-tight seal and preventexposure of the electrolytes to ambient humidity during the conductivitytesting. Once removed, the cell was placed inside a Cincinnati Sub-ZeroMicroclimate 1.2 benchtop environmental chamber. The conductivity probewas connected to an Agilent 4338B Milliohmmeter for measuringresistance. Conductivity was determined from dividing the provided cellconstant (0.8 cm⁻¹) by the measured resistance.

The ionic conductivity of selected electrolytes was evaluated incomparison with baselines over a wide temperature range, and data ispresented in FIG. 3. The graph of the ionic conductivities as a functionof temperature is shown in FIG. 2. The ionic conductivity was evaluatedfrom −40° C. to at least +60° C. or higher for specific compositions.The addition of 10 wt % FRA-D was shown to slightly decrease the ionicconductivity compared to the baseline electrolytes (either CE-C orF-030-B). The baseline (CE-C1) has very poor performance at −40° C. andadding 10% of the flame retardant (FRA-D) further decreased lowtemperature performance.

TABLE 9 Ionic conductivity values over wide temperature ranges. 80° C. ±75° C. ± 60° C. ± 40° C. ± 25° C. ± 20° C. ± 0° C. ± −20° C. ± −40° C. ±Electrolyte 2° C. 2° C. 4° C. 2° C. 1° C. 2° C. 3° C. 3° C. 1° C. CE-C120.58 — 16.34 12.78 — 8.83 5.43 2.60 0.87 CE-C1 + 18.68 — 14.44 11.19 —7.55 4.45 1.98 0.58 10% FRA-D F-030-B-03 — 18.46 16.00 12.91 10.59 9.856.90 4.06 1.65 F-032-D-03 — 16.48 14.21 11.32  9.25 8.57 5.93 3.38 1.27(10% FRA-D) F-040-D-02 — 14.62 12.69 10.22  8.36 7.75 5.34 3.05 1.16(15% FRA-D)

Using the wide temperature composition F-030-B, the addition of FRA-D ineither 10 wt % (F-032-D) or 15 wt % (F-040-D) resulted in electrolytewith very good room temperature conductivity and low temperatureconductivity. The conductivity at −40° C. for the F-040-D with 15 wt %FRA-D as a flame retardant additive is higher than the conventionalelectrolyte (CE-C1). This shows that proper combinations of additivescan result in an electrolyte with reduced flammability and improved lowtemperature performance. Adding compounds such as methyl butyrate inspecific amounts improves the low temperature ionic conductivity offlame retardant electrolytes.

Example IV

The effect of the flame retardant additives on cell performance iscritically important. Electrolyte containing flame retardant additivesand additional compounds were evaluated in cells. The performance of theelectrolyte was determined from galvanostatic cycling tests andelectrochemical impedance spectroscopy measurements.

The electrolyte was with cells containing a nickel cobalt aluminum (NCA)cathode and a graphite anode. The cathode material consisted of ˜80%nickel cobalt aluminum (NCA) cathode powders (LiNi_(x)Co_(y)Al_(z)O₂)coated single-sided on aluminum foil. The anodes consisted of ˜95%graphite powders coated single-sided on copper foil. The additionalnon-active material consisted of a conductive additive and a binder.Packaged CR2032 coin cells were fabricated as half cells and full cells.For half cells (using Li metal), the assembly in the coin cells consistsof two electrodes (Li metal and cathode or anode), a separator (Celgard2500), one or more stainless steel spacers, and a nickel foam pellet.The nickel foam was used as a spacer and to provide compression of theelectrodes and separator and to reduce the distance between theelectrodes. After assembly of the electrodes and spacer, the electrolytewas added to the assembly and the coin cell was sealed. A similarprocedure was used for preparing full cells where Li metal is replacedwith the graphite anode. Galvanostatic charge-discharge measurementswere performed on an Arbin Instruments battery test station.

Two-point probe electrochemical impedance spectroscopy (EIS)measurements were obtained using a Princeton Applied Research (PAR)VersaStat MC potentiostat operated with VersaStudio software. An appliedroot mean square amplitude of 50 mV was used over the frequency range of1.0 MHz to 0.1 Hz, and data was taken at open circuit potential ineither the charged or discharged state. Temperature was controlled witha Cincinnati Sub-Zero Microclimate 1.2 benchtop environmental chamber.For the NCA cathode half cell tests, the voltage window used was 3.0-4.1V vs. Li. For the graphite anode, the voltage range was 0.0-1.5 V vs.Li. Full cell tests used the same voltage range as used for the cathodehalf cells. Specifically, upon completion of full cell fabrication, thecoin cells typically showed on open circuit potential of ˜0 V and thuswere immediately placed on the Arbin test station to undergo a brief‘bump’ charge at C/10 rate to 2.5V. After the initial ‘bump’ charge, thecells were allowed to rest for 12 hours to allow the electrodes andseparator membrane to be fully wetted. Once the 12 hour rest period wascompleted, the cells underwent 5 formation cycles with one cycle beingdefined as a charge step to 4.1V and a discharge step to 3.0V at a rateof C/10, with 5 minute rest steps between each charge and discharge andmonitoring open circuit potential (OCP). Upon completion of formationcycles, the cells were used for further testing. For anode half cells,the open circuit potential after fabrication was typically between 1.3Vto 1.7V, thus no ‘bump’ charge was necessary. The OCP would increase tobetween 1.5V to 1.8V during the 12 hour rest period. After the restperiod, cells underwent 5 formation cycles with one cycle defined as acharge to 0.0V followed by a discharge to 1.5V at a rate of C/10, with 5minute rest steps between each charge and discharge, and monitoring OCP.After formation cycles were completed, the cells were further tested.

Electrochemical impedance analysis of full cells was performed todetermine the effect of different electrolyte compositions on the cellresistance. The affect of amount of flame retardant additive, FRA-D, onthe impedance was determined. The cells were tested in a dischargedstate (˜3.0V) after 5 formation cycles. As shown in FIG. 4, the amountof FRA-D up to 20 wt % has no negative effect on the high frequencycharge transfer resistance, and at 10% and 15% FRA-D additive amount mayslightly decrease the charge transfer resistance. This data supportsthat the interfacial charge transfer resistance is not negativelyaffected by the addition of the flame retardant additive atconcentrations of 10-20 wt % within the specific electrolyte compositionevaluated. Therefore, the interface is very similar to the baselinecomposition without the flame retardant additive. This desirable resultsupports that the electrolyte containing the flame retardant additivewill provide high performance and not significantly increase theinterfacial resistance of the cell.

The ability of the electrolytes containing the flame retardant compoundsto provide good electrochemical performance is also supported bygalvanostatic charging and discharging experiments. The effect ofdifferent additives including the flame retardant additive (FRA) andother wide temperature additives (WTAs) on the formation cycles is veryimportant. The formation cycles are the initial cycles during which timethe solid electrolyte interface layer forms. Additional plateausobserved during the formation cycles would indicate that a differentinterface layer may be formed from the compounds.

FIG. 5 shows the first formation cycle charge of cells with variouselectrolytes, indicating the WTA (ethyl propionate) and FRA (FRA-D) donot negatively affect the cell performance. FIG. 6 shows the sameinformation, however it includes additional cells. In all cells, somevoltage fluctuations were observed and attributed to variations in thetemperature of the room; however the additional cells shown in FIG. 6displayed greater temperature fluctuations. FIG. 7 shows the firstformation discharge cycle of cells with various electrolytes anddisplays the expected typical capacities of ca. 160 mA/g. Again somefluctuations are observed and attributed to temperature variance.Overall, the initial formation shows no indication of a negativeperformance influences by the WTA or FRA.

Full cells were cycled at 1 C for 100 cycles to determine the effect ofelectrolyte composition on cycle life. In FIG. 8, cycle life testing ofcells with various electrolytes is shown. Cells underwent 3 formationcycles at C/10 (not depicted) and were then cycled at 1 C rate for 100cycles. This test shows the WTA, ethyl propionate, may induce someattrition to cycle life of the cell, however, the FRA seems tocounteract this process and diminish effects of the WTA.

To test alternative FRAs, additional cycle life testing at 1.0 C rateand room temperature conditions was performed and shown in FIG. 9. It isevident that electrolytes with TMP (FRA-C) and DMMP (FRA-B) additivesshow inadequate cycle life, and the electrolyte with TTFEP (FRA-A)additive does not perform as well as electrolytes with FRA-D.

Example V

The low temperature performance of cells can be improved by usingspecific components in the electrolyte. In Example II, the ionicconductivities of electrolytes that were flame retardant were shown tobe improved at low temperatures by adding compounds such as methylbutyrate or ethyl propionate. The cell performance at low temperaturealso demonstrates that specific types and amounts of additives improvethe low temperature performance of flame retardant electrolytes.

Cells containing different electrolytes and tested at −40° C. are shownin FIG. 10. Cells prepared with F-007-D-01 (with WTA, ethyl propionate,and 10 wt % FRA-D) are shown in orange, and cells prepared withF-008-D-01 (with WTA and 20 wt % FRA-D) are shown in Violet. Cells withaddition of the WTAs showed both a lower operating voltage and a reduceddischarge compared to cells tested using the commercial electrolyte withno WTA. However, addition of the FRA into the electrolyte showedslightly increased performance over the Lynntech baseline with only theWTAs, again indicating the combination of WTAs and FRA is better thanthe only WTAs.

Data shown in FIG. 10 shows that electrolytes F-032 and F-040 showedhigher performance at −40° C. compared with baseline (CE-C) and theprevious composition (F-007), and the electrolyte F-032 showed the bestlow temperature performance, and that overall performance at lowtemperatures is positively affected by addition of the FRA-D additive.The electrolytes F-032 and F-040 contained specific amounts (ca. 38-41%)methyl butyrate in addition to the other components. The specificcombination the type and amount of lithium salt, solvents (EC, EMC andMB), flame retardant additives, and solid-electrolyte interface formingcompounds provided reduced flammability and wide temperature rangeoperation.

Electrolytes containing a low freezing point compound such as methylbutyrate, combined with the specific flame retardant additive and solidelectrolyte interface-forming compound provide good low temperatureperformance. The performance of flame retardant compounds can be similaror better than baseline flammable components through a combinationspecific amounts and types of salt, solvents, low freezing pointco-solvents, flame retardant additives and solid electrolyteinterface-forming compounds.

The two best compositions (F-032-D and F-040-D) for low temperatureperformance show better performance than the baseline electrolyte andimpart lower flammability. It is evident that 20% FRA-D additive showslower performance, most likely due to a decrease in conductivity. Thisshows the inherent trade-off and the need to optimize the specificamount of flame retardant compound in order to obtain optimal lowtemperature performance.

Example VI

Specific amounts of solid electrolyte interface forming compound areneeded in order to enable both low temperature performance and hightemperature stability. To determine the effect of the amount ofinterface forming compound on the high temperature stability, full cellcoin cells were evaluated at +71° C. using the lithium nickel cobaltaluminum oxide (NCA) cathode and graphite anodes, as described inExample IV.

Tests performed used a commercial electrolyte composition (CE-C) andseveral Lynntech electrolyte compositions (F-006-B, F-030-B, etc.) as abaselines to compare electrolytes with WTA/s and/or FRA additives.Compositions are as described in Example I. The baseline of any givenset of data is represented in blue, unless noted otherwise. Full cellsfirst undergo five formation cycles (unless noted otherwise) at a rateof C/10 and at room temperature. Then cells were cycled by charging anddischarging at a rate of 1 C at +71° C.

Shown in FIG. 11 are cells prepared with (1) the baseline CE-Celectrolyte, (2) F-007-D-01 (with WTAs and FRA) and (3) F-015-01 whichincludes no primary WTA (VC), but three alternative WTAs for high tempperformance and the FRA. Cells with addition of the WTAs showed improvedperformance over cells with the commercial baseline electrolyte.However, the electrolyte with the primary WTA out performs theelectrolyte with multiple secondary WTAs for high temperatureperformance. While the WTAs show improved performance of the baseline,there is a loss in overall discharge capacity in all cells over 30cycles.

Based on these results, an additional evaluation was performed todetermine the effect of WTA-A (VC) on high temperature performance.Electrolytes with various amounts to WTA-A were evaluated and comparedagainst baselines, as shown in FIG. 12. Clearly all the electrolytesout-performed the baseline (CE-C; blue), but a very noticeable trendoccurs showing that with increasing amount of WTA-A improves hightemperature stability. The composition F-007-D showed the best hightemperature performance; however, the poor low temperature performancediscussed below hinders the use of this composition for wide temperatureranges. Clearly, even a small amount of WTA-A increases high temperatureperformance, most likely due to reactions occurring at the electrodes toprovide a SEI layer that protects the cell in elevated temperatureregimes; though as discussed above, that same additive, even in smallamounts, greatly increases overall cell resistance and decreases cellcapacity at low temperature regimes. From this evaluation, specificamounts of WTA-A in the range of about 0.5 to about 2.0 wt % can improvehigh temperature stability. An intermediate amount of VC in the range ofabout 0.5 to about 1.0 wt % also provides improved high temperaturestability.

In order to evaluate the effect of FRA-D on high temperatureperformance, electrolytes with various amounts of FRA-D additive wereevaluated and are shown in FIG. 13. From this data, the percent FRA-Dadded to the electrolyte (up to 20 wt. %) shows no negative effect tocell performance at elevated temperatures. One possible explanation forstable performance with increasing FRA-D additive is that FRA-D may playa role in formation of a better SEI layer on the electrodes than thebaseline electrolyte without FRA-D.

The amount of the wide temperature additive, (vinylene carbonate, VC,WTA-A) was varied and its affect on the cell resistance was probed usingelectrochemical impedance spectroscopy (EIS) at room temperature over afrequency range of 1 MHz to 0.1 Hz. Cells were tested in a dischargedstate (˜3.0V) after 5 formation cycles. The amount of WTA-A (VC)affected the high frequency charge transfer resistance as shown in FIG.14. Lower VC amounts resulted in lower cell resistance, and higheramounts of VC resulted in higher resistance. This supported that anoptimal amount of VC is needed to minimize resistance while alsoproviding a protective layer to prevent electrolyte decomposition atelevated temperatures.

The effect of weight percent addition of the WTA-A (VC) on lowtemperature performance was determined. In FIG. 15, a clear trend isformed showing a decreasing cell voltage with increasing amount ofWTA-A. The additions of WTA-A are 0 wt % (orange), 0.5 wt % (violet),1.0 wt % (red), and 2.0 wt % (black). Additionally, these electrolytesare compared to the commercial baseline, CE-C (blue; 0 wt %), and theelectrolyte, F-007-D (green; 1.8 wt %). Electrolytes F-032-D and F-037-Dshowed the best overall performance at −40° C. compared with thecommercial baseline (CE-C) and the previous composition (F-007-D).Therefore, there exists a specific amount of WTA-A (VC) in the range ofabout 0.5 to about 2.0 wt % with 1.0 wt % being the most optimal toprovide both good low temperature performance and high temperaturestability. A sufficient amount of additive VC is needed to allow astable interface layer that protects the electrolyte and electrodes athigh temperatures from degradation, however too much VC results in ahighly resistive layer that reduces low temperature performance. Otherthan the baseline electrolytes, all the electrolytes shown in FIG. 15had 10 wt % FRA-D. The data supports that specific electrolytecompositions containing the proper amount and type of compounds canprovide reduced flammability and similar low temperature performance asbaseline flammable electrolytes.

The order of execution or performance of the methods and processesillustrated and described herein is not essential, unless otherwisespecified. That is, elements of the methods and processes may beperformed in any order, unless otherwise specified, and that the methodsmay include more or less elements than those disclosed herein. Forexample, it is contemplated that executing or performing a particularelement before, contemporaneously with, or after another element are allpossible sequences of execution.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is, therefore, intended that the appended claimsencompass any such modifications or embodiments.

What is claimed is:
 1. An electrolyte for an electrochemical storagedevice, the electrolyte comprising: a lithium salt from about 3% toabout 20% by weight; a primary solvent from about 15% to about 25% byweight; wide-temperature co-solvents from about 14% to about 55% byweight; interface forming compounds from about 0.5% to about 2.0% byweight; and a flame retardant compound from about 6% to about 60% byweight, wherein the flame retardant compound is2-ethoxy-2,4,4,6,6-pentafluoro-1,3,5,2λ5, 4λ5, 6λ5 triazatriphosphinine,wherein the electrolyte interacts with at least one positive electrodeconfigured to store and release lithium-ions and at least one negativeelectrode configured to store and release lithium-ions, the at least onepositive electrode and the at least one negative electrode formingportions of the electrochemical storage device and at least one of thesolvent or co-solvent comprises methyl butyrate.
 2. The electrolyte asrecited in claim 1, wherein the electrochemical storage device furthercomprises a device selected from the group consisting of batteries,lithium-ion batteries, and supercapacitors.
 3. The electrolyte asrecited in claim 1, wherein the electrochemical storage device furthercomprises a lithium-ion battery.
 4. The electrolyte as recited in claim1 wherein the lithium salt further comprises a salt selected from thegroup consisting of lithium hexafluorophosphate, lithiumbistrifluoromethanesulfonimide (LiTFSI), lithium tetrafluoroborate(LiBF₄), and combinations thereof.
 5. The electrolyte as recited inclaim 1, wherein the lithium salt further comprises lithiumhexafluorophosphate.
 6. The electrolyte as recited in claim 1, whereinthe electrochemical storage device further comprises a lithium-ionbattery.